1. Field of the Invention
This invention relates to improvements to microwave transmission lines comprising two flat parallel and coplanar conductive strips.
2. Description of the Prior Art
Such normally used transmission lines are divided into two types, those referred as to symmetrical lines and those referred to as asymmetrical lines. A symmetrical line consists of two linear metal strips having equal widths W and arranged parallel to one another at a predetermined distance G on a non-conductive substrate. An asymmetrical line consists of a first conductor in the form of a narrow flat metal strip having a small width W and a second conductor in the form of a wide longitudinal conductive area or strip having a width l much greater than W and placed parallel to the narrow conductive strip at a distance G therefrom on the same type of substrate.
For a given characteristic line impedance, the symmetrical line requires a ratio W/G, width of strip over width of interstice between conductors, greater than that of the asymmetrical line. The result of this is that the symmetrical line has wider strips than that of the asymmetrical line and/or a narrower interstice than that of the asymmetrical line. This dimensional feature of the asymmetrical line is advantageous in that it makes use of less resistant conductive strips while reducing line width. The symmetrical line is often chosen when it is necessary to provide symmetry of the electric and/or magnetic fields of the microwave that is propagated in the line.
However, two major drawbacks inherent in the connection of the line and in the resonances of the line are to be considered when a symmetrical line is used.
In general, the use of the symmetrical line requires connections between ends of the line and exterior microwave components such as a microwave source, load, or probe, by means of miniature or subminiature coaxial connectors. As already known, such a coaxial connector comprises an elongate central internal conductor having a small diameter and a cylindrical external conductor having a greater diameter and, consequently, offers an asymmetrical conductive structure. The differences in geometric shapes of the connector and the symmetrical line also give rise to difficulties with connection. In practice, these difficulties are resolved by providing, at the end of the line to be connected, a small substantially rectangular flat end conductor connected coplanarly to the end of the one of the linear strips and forming with the end of the other strip a portion of a flat asymmetrical line. The end conductive plane is laterally welded to the external cylindrical conductor of the coaxial connector, and the projecting end of the internal conductor of the connector is welded to the end of the other strip of the line.
The second drawback of the symmetrical line consists in the appearance of relatively low spurious freuqencies of longitudinal resonance which limit the useful frequency band of the symmetrical line. The longitudinal resonances are by definition lower than transverse resonances that are within the very high frequency range. Experimental analysis of resonance shows that some of the microwave energy is neither transmitted nor reflected, but is radiated. In fact, a symmetrical line has natural frequencies for which a stationary wave may be formed, thus setting up a source of radiation.